A Robust Distance Protection Approach for Bulk AC Power System Considering the Effects of HVDC Interfaced Offshore Wind Units

Offshore wind generators are connected to the ac power grid through high-voltage direct current (HVDC) transmission systems. However, when a short-circuit fault occurs in a transmission line, which is connected to the point of common coupling (PCC) of the HVDC system, the performance of distance relays on the ac grid is affected. This is due to rapid response of reactive power control of the grid-side voltage source converter (GVSC) of the HVDC system to compensate the voltage drop of the PCC bus. In other words, during the fault, the GVSC injects the reactive power to the ac grid to restore the PCC bus voltage. This in turn causes to interfere with the distance protection scheme by affecting the viewed impedance of distance relays. To overcome this problem, based on robust control theory, the present paper introduces a robust control approach for voltage source converter of HVDC system using μ synthesis analysis. The proposed strategy controls the reactive power injected to the PCC bus by trying to keep the defined current ratio $\alpha $ constant. The effectiveness of the proposed approach was validated through simulation using MATLAB/Simulink.